The invention relates to a method for controlling an internal combustion engine.
International publication WO 97/36762 discloses a method for controlling a four-stroke internal combustion engine, in which method a desired torque for an internal combustion engine is calculated from a maximum value and a minimum value for an engine torque and from a weighting factor. The weighting factorxe2x80x94also designated as the torque factorxe2x80x94is derived from an accelerator pedal setting and a current rotational speed of the internal combustion engine. The values for the maximum and minimum torque are determined depending on the rotational speed, a torque loss and further operating variables. The torque loss is in turn calculated from the rotational speed and the aforementioned operating variables.
In the case of so-called lean-running internal combustion engines, it is now conventional to introduce a small quantity of fuel into the combustion chamber as a so-called pre-injection, in order to produce the most homogeneous mixture possible in the whole of the combustion chamber, particularly in the case of so-called stratified charge operation, the mixture burning reliably and completely during the working stroke. It has also become conventional to introduce a small quantity of fuel, the so-called post-injection, into the combustion chamber when the exhaust valves are already open. This post-injected quantity of fuel then burns in the exhaust manifold and causes a significant increase in temperature of the emitted combustion exhaust gases. A catalytic converter which is located in the exhaust train should thus be brought more quickly to its required operating temperature for optimum purification effect, particularly in a start phase of the internal combustion engine.
In the known method for controlling internal combustion engines by determining torques, it has not previously been possible fully to take into consideration these additional injection pulses in the relevant working strokes of the four-stroke internal combustion engine.
The invention discloses a reliable determination of the torque of an internal combustion engine in which a plurality of injection pulses take place during each working stroke of the internal combustion engine.
In accordance with one embodiment of the invention, there is a method for controlling an internal combustion engine takes both a pre-injected and a post-injected quantity into consideration when calculating a current torque, by introducing an additional factor in each case. Depending on the phase position of the pre-injection or post-injection to the main injection, the factor is multiplied by the current torque for the main injection, and therefore reflects the effect of these additionally injected fuel quantities on the combustion pressure and therefore on the engine torque.
In another embodiment according to the invention, there is a method for controlling an internal combustion engine which has a fuel injection system, in which each fuel injection pulse for a combustion chamber during a working stroke can include at least one additional quantity of pre-injected and/or at least one additional quantity of post-injected fuel in addition to a main injection quantity at the intended injection instant, provision is made for including at least the following components in a control system which is assigned to the internal combustion engine: a pedal value sensor for capturing a current accelerator pedal value and a rotational speed sensor for capturing a current rotational speed of the internal combustion engine. Provision is also made for determining a preferred torque for the clutch-side end of a crankshaft, based on the accelerator pedal value and on the rotational speed, and for deriving from the preferred torque at least one actuating signal for at least one actuator of the internal combustion engine. Such an actuating signal can preferably be a throttle valve servomotor or similar.
In accordance with another embodiment of the invention, in the case of a pre-injected fuel mass which is supplied to the combustion chamber at an earlier instant than a main-injection mass, a correction to the torque-determining total fuel mass is carried out, wherein a value for a time or phase difference between injection start or injection end of the pre-injection and injection start or injection end of the main injection is taken into consideration in an assigned correction factor. Accordingly, in the case of a post-injected fuel mass which is supplied to the combustion chamber at a later instant than the main-injection mass, a correction to the torque-determining total fuel mass is likewise carried out, wherein a value for a time or phase difference between injection start or injection end of the main injection and injection start or injection end of the post-injection is taken into consideration in an assigned correction factor. The correction factor typically takes into consideration the injection start of the different injection pulses in each case. However, it is also possible to take into consideration, for example, the injection end of the pre-injection and the injection start of the main injection.
In a preferred embodiment of the invention, provision is made for additionally capturing a current value for the air mass flow in the control system which is assigned to the internal combustion engine. Since a fuel mass flow has not previously been directly measured in conventional systems, the current value for the fuel mass flow, which represents a relevant measurement variable for the method, is derived from the air mass flow, with reference to a characteristic map if applicable.
In addition to the driver preference, which is derived from the current position of the accelerator pedal, known control methods include further torque-determining or torque-requiring influence variables. Such an influence variable can be, for example, a torque requirement of a travel speed regulator which is known per se, or a torque requirement of an anti-slip regulator (ASR) which is known per se. An ASR system typically includes the capture of a current wheel rotational speed and a comparison with an actual speed of the vehicle. In the case of significant variations, a reduction of a desired torque is generated, together with a braking intervention on the individual wheel if applicable. A further torque requirement can be generated, for example, by a so-called ESP system (xe2x80x98electronic stability programxe2x80x99). These torque-determining influence variables are preferably taken into consideration by means of assignment rules from characteristic maps.
In another preferred embodiment in accordance with the invention, an overall ratio of air and fuel per working stroke is taken into consideration for the correction of the determined total fuel mass. When calculating a current torque, consideration is preferably given to a correction factor which comprises the sum of at least two of three factors F1, F2 and F3. In this case, the first factor F1 is a function of the fuel mass supplied in the main injection: F1=f1(mfuel,main); the second factor F2 is a function from the factor of the fuel mass supplied in the pre-injection and of a first efficiency level xcex72, which depends on the time separation between pre-injection and main injection: F2=f2(mfuel,pre)* xcex72(phasepre-main), expressed here by the term xe2x80x98phasexe2x80x99. This phase can be a time-related variable or express an angular relationship which specifies an angular difference in degrees between pre-injection and main injection. The third factor F3 is finally a function from the factor of the fuel mass which is supplied in the post-injection and of an efficiency level xcex73, which depends on the time separation between main injection and post-injection: F3=f3(mfuel,post)*xcex73(phasemain-post). In summary, it follows that:
Feff=f1(mfuel,main)+f2 (mfuel,pre)*xcex72 (phasepre-main)+f3(mfuel,post)*xcex73(phasemain-post). 
In still another preferred embodiment in accordance with the invention, the remaining oxygen share xcex in the exhaust gas can also be considered in the efficiency levels xcex72 and xcex73 for calculating the torque, with the result that the relationship is extended as follows:
Feff=f1(mfuel,main)+f2(mfuel,pre)*xcex72(phasepre-main,xcex)+f3 (mfuel,post)*xcex73 (phasemain-post, xcex) 